Chapter 21      Conclusion

 

      “Give a good skipper any boat that is reasonably well set up and he will win.”

 

It may seem to be a strange way to open my concluding chapter with a quotation. It was said to me before I started this book but, whilst all of us who race model yachts will recognise that it is a valid observation, I needed to write this book to find out why. On the face of it the quotation appears to say that it is the skill of the skipper that matters and not the design and tune of the boat. But there is more to it than that. Just recently one member of the Swanley club has been able to race on a large lake with no obstructions round it. His important observation after racing a few times was that little distance covered the yachts at the end of a race compared with racing at Swanley, where there are lots of lakeside obstructions, and over six legs boats may be two legs apart. As yachts are of different designs and sailed by skippers of different levels of skill one can only conclude that the performance of all the yachts was very much the same. This is just what the physics of model yachts appears to show.

 

Let us look at the yacht as a whole and see just how this comes about. We started by separating the yacht into three components having three different jobs, the hull, the fin and rudder, and the rig. We now need to assess their performances as part of a racing yacht and how, in the end, they work together.

 

The sailing rig

The sailing rig is made up of the sails and the spars and cordage. The main sail and the mast act together as one device to divert the flow of air. The fore sail normally has no spar at its luff to affect its shape and is then just a single curved surface. The two sails act together to produce a net force which has a component which can drive the yacht. The other spars and the other wires and cords have their own function but they offer drag which may add to the drive downwind but they also cause drag to slow the yacht when it is reaching or beating.

 

As we have seen sails work in one of two ways. When the yacht is running the flow is square on to the sails and simply flows up to and around the sails as it does round a kite. When the yacht is reaching or beating the air flows across the sails and is made to change its direction.

 

When a yacht is running energy is extracted from the wind by slowing it down but the flow over and round the sails is hopelessly chaotic. It is hard to see any design process by which the performance of the sails when running can be improved. The flow is so unpredictable that the all we can do is set the sails without twist and put up with the result. Then we might reasonably expect that the force on the sails will increase more or less in proportion with the area of the sails provided that both sails are exposed directly to the wind as in the case for a yacht running goose-winged. We have to accept that, when running, the force on the rig depends on the difference between the speed of the wind and the speed of the yacht. This means that an equilibrium speed is established quickly. The most significant advantage of the swing rig is the fact that it always runs goose-winged so that the maximum area is presented to the wind.

 

When a yacht is beating or reaching the air flows across the sails as it does for an aerofoil. If only we could use a rigid aerofoil at low angles of attack we might be able to take advantage of lift/drag ratios in the twenties but the soft sail used on model yachts cannot work at low angles of attack because the sail deflates. Instead it must work at angles of attack of about 30° with lift/drag ratios of 2 or 3. This is the great weakness of the soft sail. It is a limitation that is insurmountable and it is compounded by the natural veering of the wind and the additional effect of lakeside obstructions. Fortunately Diagram 8-6 is not the end of the story because, as we have seen, the diversion of the flow ahead of the mainsail when beating lets us put the fore sail in a favourable position for it to generate lift and in turn to drive the yacht. Detail changes in the rig cannot offset these major causes of poor performance.[1] We simply cannot avoid generating a wide diffuse wake with both sails.

 

The hull and the rig

It is obvious that the function of the rig is to drive the hull and the under water parts through the water. The rig acts on the wind to produce a net force that has components acting sideways and forwards on the hull. The fin and rudder are there to deal with the transverse force and usually they are very efficient. The hull moves so that the resistance to its motion through the water is equal to the net forward force generated by the rig. We have seen that the behaviour of the water as it flows round the hull is typical of all sorts of floating bodies, even ducks. We have also seen that this flow pattern leads to the speed versus thrust relationship that is shown in Graph 7-6. Designers of hulls for model yachts all hope to end up with a hull that moves more quickly for the same thrust. Long thin hulls such as are used on naval vessels and on fast, power driven displacement hulls will move very quickly but they are not much good for carrying sail. In most cases the class rules impose restrictions that block experiments along this line. We are stuck with our chubby displacement hull.

 

Given this restriction, we have seen that the shape of the hull may vary widely but, other than the fact that some hulls are better suited to particular conditions, there is no single shape that is so good that it has superseded all the others. We can see why if we go back to Graph 7-6 which is typical for model yachts. Over the range of thrust from 0.1 to 0.3 pounds the graph is more or less straight and an increase in thrust of 10% only produces an increase of about 3% in yacht speed.

 

Suppose that we had two yachts of the same class racing in a steady wind sufficiently far apart for there to be no interference between them. Each would be set up well enough for racing. Class regulations are designed to make the rigs produce the same drive so that the likelihood of one rig being able to produce 10% more drive than the other is remote. Our graph tells us that, any small difference that one may enjoy over the other, is divided by three when it appears as an increase in boat speed. There is always the possibility that one hull is easier to drive than another but all the evidence suggests that whilst this may be true in a particular combination of wind and wave it will not be true over the whole range of combinations. The factors that work to determine the speed of a model yacht tend, for each class, to produce the same speed through the water for a given wind speed and course relative to the wind. We have to accept that even if we can think up a way to improve the performance of the sails the hydrodynamic performance of the hull will reduce the effect to insignificance. However we can impair the performance of the rig quite easily by introducing unwanted drag which has to be overcome and so reduce the drive going to the hull.

 

It is instructive to digress to land yachting for a moment. Land yachts use the same rig as waterborne yachts but they run on wheels over a level surface. As they do not have to climb hills the resistance to motion on, say a reach, is primarily that caused by the wheels. This starting resistance is quite high but once the wheels are rolling the resistance increases more or less in proportion to the speed of the yacht but not very quickly. There is nothing in rolling resistance like the rise in hydrodynamic resistance caused by the growth of the bow wave. As a result land yachts achieve very high speeds when compared with waterborne yachts.

 

However it is the sailing rig that really settles the matter. It is intrinsically inefficient which means just what it says, we cannot improve it beyond a certain level of performance.

 

Sailing in light winds.

Text Box:  
Graph 21-1
In Graph 21-1 I have taken Graph 7-6 as the basic graph and then chosen to represent the mathematical   line approximately by two straight lines. They were fitted by eye. In doing this I have separated the whole range of speed of the yacht into two ranges each represented by a straight line. The lower range of speeds will occur in light winds and the other range in anything from a breeze to a stiff wind. If we suppose that the drive is proportional to the square of the wind speed we can make an estimate of the ratio of wind at the changeover point to that at the critical yacht speed. It is about 3 to 1. If the rig can work up to 10 mph (which is probably too much) then the winds in the low range are less than 3 mph. Weather forecasters do not recognise such light winds as the effects of thermal winds are more than this. Nevertheless yachts have to be sailed in such conditions.

 

I drew the polar performance diagrams for a wind of 10 mph and a critical speed of the yacht of about 3 mph. In the range for light winds the wind speed might be 1 mph and correspond to a yacht speed of the order of 0.5 mph. This will produce a new polar diagram with a much greater range of angles between the true wind, the relative wind and the course of the yacht. If a mast head burgee can be made to work it is the only way we have of trying to set the rig to the best position.

 

The model yacht as a whole.

This combination of parts, some of which work well and some not so well, produces a model yacht that is fun to race. The fact that the rig is inefficient means that, in order to reach the maximum speed in a typical wind the rig must have a large sail area, and it must be quite tall. This in its turn calls for a significant weight of lead to be attached somewhere on the yacht just to maintain stability. This weight gives the yacht sufficient momentum at the low speeds of model sailing to let it tack through head to wind in most sailing conditions. If this were not the case monohull yachts would be as difficult to race as multi-hulls. The fact that the rig is unavoidably inefficient means that no-one can get round the problem and ensures that model yachts built to reasonable standards can always be sailed competitively with a chance of winning. Then the consistent winners are those who can sail best and employ the rules of racing to the greatest advantage. The eternal optimism of the ordinary sailor ensures that innovations in the rig will be explored enthusiastically

 

 

The design guidelines for model yachts

 

We have seen that the underwater parts of model yachts are already quite well designed and it is unlikely that fairings on fins or fancy shapes will improve anything. Certainly most ordinary club sailors will find a yacht with generous areas for the fin and rudder to give less sensitive control. A top quality finish is desirable and this should extend to the lead. No serious sailor would leave a yacht grounded on its lead during a tea break.

 

There are dozens of different designs for the hull for the popular classes. No one of these designs has come to be better than the others. All one can say is that hulls should have smooth curves in every direction and of course a first class finish.

 

The rig is another matter. However it is designed any drive that may be created by the sails must not be wasted in overcoming friction losses before it is used to drive the yacht through the water. If one regards the mast as part of the mainsail then the items associated with the mainsail that cause unwanted drag are the shrouds, the spreaders, the mast head crane, the back stay and the boom. It would be best if we could do away with all of them. As most designers want to retain them it should be recognised that round wire has a higher drag coefficient than a plate held square to the wind. Wires and cords of the smallest possible diameter should be used and bowsies and cordage tucked away inside booms or along booms if at all possible. A boom on edge has a greater drag than a boom that is flat. The foresail is best if the leach line can be omitted as for an A boat but, where it must be used, as for a metre boat, it should have the minimum diameter.

 

A sailing rig has to work with a continually veering wind and it seems to follow from aeronautical practice that a round luff would respond least to this fluctuation. This means using a pocket luff on the fore sail. The mainsail already has the mast to act as its round luff but the use of a pocket luff is not easy to arrange. Attention to detail on mast rings may well pay dividends.

 

We have seen in Figure 18-7 that the rig and the fin have to be properly placed relative to each other but have said nothing about the position of these two relative to the hull. There seems to be a marked agreement on the best position for the mast (and therefore the fin) but it does not seem to be critical. If it were to be critical we would have to make provision to move the mast and the fin for a new design. Mostly designers fix the fin (probably with reference to other yachts) and possibly provide an arrangement to move the mast fore and aft that, once set, is not adjusted again. There is an incentive to avoid this complication, and the permanent addition of unwanted weight to the yacht, by the purchase of well-tried designs.

 

Post script

I completed this book ten years ago and I have learnt a great deal more about the practical problems of sailing in those years.

 

I think that that the most important aspect of sailing, that I recognised in 1999 but did not fully appreciate, is the problem of sailing in the confused wind that results from the wind flowing round obstructions like trees and buildings. I sailed at Swanley during a time when the trees were maturing and the wind gradually deteriorating to become confused over most of the lake. It was not until I had given up yacht racing and I took my A boat to a lake with a steady wind that I realised just how good a well set up A boat can be. It is a sheer joy to sail it.

 

So this book turns out to be about the way in which a yacht sails in a steady wind. Even if I had set out to explain how a model yacht should be designed to sail in a confused wind I would have needed to look at how it sails in a steady wind first. If you accept this limitation then I do not think that I want to change the main thrust any of it but I think that I might change the emphasis here and there if I were to be re-writing it.

 

Clearly I did not address the question of how one might sail for the best in a confused wind. The real answer is “with a different boat” because the fore and aft rig with triangular sails does not lend itself to adaptation. Since in model yacht racing a change of boat is not an option one must ask how one might sail an ordinary model racing yacht in confused air.

 

I think that the first thing to do is to make “paddling” permissible so that it becomes possible to turn at marks by changing course when stationary. This is already tacitly in operation but with some restriction on the manner of paddling. It requires a change in the design of the rudder to make it unbalanced. Then there is a nice design problem in choosing a throw for the rudder. Too much throw and it is difficult to sail a smooth course, too little and you cannot paddle. Paddling is a new skill to be learnt.

 

Text Box:  I never tried to set up a racing yacht for confused wind and never paddled. I preferred to travel 40 miles to a lake with a steady wind than to sail a yacht adapted for confused wind. However what I have done is built a model of a Norfolk wherry and it is a revelation. I built it because the Norfolk wherry was developed for exactly these conditions of swirling air from riverside trees and buildings and it was developed to sail on rivers where tacking is impossible. This last gives a great incentive to get close to the wind and so reduce the distance to be poled. The wherry has the longest unbalanced rudder on any sailing boat. It uses a gaffe rig on which the gaffe is very long (about as long as the luff) and can be raised or lowered to give the single, loose-footed sail a large twist and some belly. It has a single stout mast that has one forestay but is un-stayed side-to-side. This gives a clean rig that goes well to windward and the sail copes with confused wind very well because whilst the twisted sail is not as good as a sail with a modest twist in good air it always has some part of the sail driving in a confused wind. In competition I sailed it 13 times round a big course in very disturbed air on a lake surrounded with mature trees and did not tack once. If I were to return to model yacht racing I think that I might try to set up my yacht to give it the shape used on the wherry.

 

I made a new cotton main-sail for my A boat to incorporate a radio controlled adjustable gaffe and it goes well. It is interesting to adjust that gaffe in steady wind as well as unsteady wind. Perhaps it is not without significance that modern full-size yacht designs are using short gaffes at the head of the main sail.

 

For a more exhaustive explanation of this problem see the Preamble to the book on the Thames Sailing Barge.



[1] Wind surfers and those who sail small yachts are the only sailors I know who can prevent their sails deflating and they do it by using a fully battened sail and they physically turn the sail “inside out” when changing tacks. Even so they have to contend with a wind that is constantly veering in a random way and whilst they can operate at lower angles of attack the angles still produce low lift/drag ratios for most of the time. Even this ploy is not open to sailors of model yachts or to sailors of larger yachts.